Inner-core anisotropy beneath Australia and differential rotation
Inner-core anisotropy beneath Australia and differential rotation
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DOI：
10.1046/j.1365-246x.2002.01780.x
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发表时间：
2002-10-01
影响因子：
2.8
通讯作者：
Nakanishi, I
中科院分区：
文献类型：
--
作者：
Isse, T;Nakanishi, I
自1986年以来，人们就提出了P波速度的内核各向异性。现在正在讨论各向异性结构的细节。我们已经研究了区域结构的内核下澳大利亚揭示的各向异性的细节使用的残差的差分旅行时的PKP（BC）减去PKP（DF）。我们收集的射线不仅几乎平行于地球自转轴，而且几乎平行于赤道平面，以区分内核的各向异性和各向同性异质性。在澳大利亚地下的内核中，收集到的射线以不同的角度相互交叉。我们假设局部均匀的横观各向同性，研究了内核各向异性。速度异常随内核射线方向与地球自转轴夹角的增大而减小，这支持了该区域内核各向异性的存在。内核顶部190公里为各向同性，速度异常为0.11%，深部为各向异性，振幅为3.1%，各向异性的对称轴线与地球表面相交，方位为北纬80度，东经103度。这一结果表明，以往的研究低估了该区域下内核各向异性的大小，由于对称轴与地球自转轴倾斜，因此，如果内核以不同于地壳和地幔的速率自转，我们可以观测到内核各向异性引起的速度异常的时间变化。考虑到澳大利亚之下的内核各向异性的区域结构的差异旋转的可能性进行了研究。本文利用南极昭和站27年来记录的地震图。尽管我们不能排除大约0.2度/年（-1）或更小的速率，但我们没有发现明显的差异旋转的证据。即使内核旋转速度比地壳和地幔快，差异也相当小，我们不需要考虑差异旋转对内核各向异性研究的影响。
The inner-core anisotropy of P-wave velocity has been suggested since 1986. The detail of the anisotropic structure is now under discussion. We have studied a regional structure in the inner core beneath Australia to reveal the detail of the anisotropy using the residuals of the differential traveltimes of PKP( BC) minus PKP(DF). We have collected the rays not only nearly parallel to the Earth's rotation axis but also nearly parallel to the equatorial plane to distinguish the anisotropy from the isotropic heterogeneity in the inner core. The collected rays cross each other with different angles in the inner core beneath Australia.We have studied inner-core anisotropy assuming locally uniform transverse isotropy. The velocity anomalies decrease with the angles between the ray directions in the inner core and the Earth's rotation axis, which supports inner-core anisotropy beneath this region. The top 190 km of the inner core is isotropic with a velocity anomaly of 0.11 per cent and the deeper part shows the anisotropy with an amplitude of 3.1 per cent. The symmetry axis of the anisotropy crosses the Earth's surface at 80degrees N and 103degreesE. This result suggests that previous studies underestimated the size of the anisotropy in the inner core beneath this region.Because the symmetry axis is inclined from the Earth's rotation axis, we can observe the temporal variation of the velocity anomalies caused by the anisotropy in the inner core if it rotates at a different rate from the crust and the mantle. The possibility of differential rotation is studied by considering the regional structure of the inner-core anisotropy beneath Australia. We use seismograms recorded at Syowa station in Antarctica over the past 27 yr. We have found no evidence for significant differential rotation though we cannot rule out a small rate of about 0.2 deg yr(-1) or less. Even if the inner core rotates faster than the crust and mantle, the difference is fairly small and we need not consider the influence of the differential rotation on the study of anisotropy in the inner core.